Chapter 2: Objects and Primitive Data

1. INF 523Q Chapter 2: Objects and Primitive Data

2. Objects and Primitive Data • Chapter 2 focuses on: • predefined objects • primitive data • the declaration and use of variables • expressions and operator precedence • class libraries

3. object method Information provided to the method (parameters) Introduction to Objects • Initially, we can think of an object as a collection of services that we can tell it to perform for us • The services are defined by methods in a class that defines the object • In the Lincoln program, we invoked the println method of the System.out object: System.out.println ("Whatever you are, be a good one.");

4. The println and print Methods • The System.out object provides another service as well • The print method is similar to the println method, except that it does not advance to the next line • Therefore anything printed after a print statement will appear on the same line • E.g.: Countdown.java (p. 53)

5. Countdown.java • //************************************************************ • // Countdown.java Author: Lewis and Loftus • // Demonstrates the difference between print and println. • //************************************************************ • public classCountdown • { • // Prints two lines of output representing a rocket countdown. • public static void main (String[] args) • { • System.out.print ("Three... "); • System.out.print ("Two... "); • System.out.print ("One... "); • System.out.print ("Zero... "); • System.out.println ("Liftoff!"); // appears on first output line • System.out.println ("Houston, we have a problem."); • } • }

6. Abstraction • An abstraction hides (or ignores) the right details at the right time • An object is abstract in that we don't really have to think about its internal details in order to use it • We don't have to know how the println method works in order to invoke it • A human being can only manage seven (plus or minus 2) pieces of information at one time • But if we group information into chunks (such as objects) we can manage many complicated pieces at once • Therefore, we can write complex software by organizing it carefully into classes and objects

7. The String Class • Every character string is an object in Java, defined by the String class • Every string literal, delimited by double quotation marks, represents a String object • The string concatenation operator (+) is used to append one string to the end of another • It can also be used to append a number to a string • A string literal cannot be broken across two lines in a program • E.g.: Facts.java (p. 56)

8. Facts.java • //************************************************************ • // Facts.java Author: Lewis and Loftus • // Demonstrates the use of the string concatenation operator and the • // automatic conversion of an integer to a string. • //************************************************************ • public class Facts • { • // Prints various facts. • public static void main (String[] args) • { • // Strings can be concatenated into one long string • System.out.println ("We present the following facts for your " • + "extracurricular edification:"); • System.out.println (); • // A string can contain numeric digits • System.out.println ("Letters in the Hawaiian alphabet: 12");

9. Facts.java (cont.) Facts.java (cont.) • // A numeric value can be concatenated to a string • System.out.println ("International dialing code for Anarctica: " • + 672); • System.out.println ("Year in which Leonardo da Vinci invented " • + "the parachute: " + 1515); • System.out.println ("Speed of ketchup: " + 40 + " km per year"); • } • }

10. String Concatenation • The plus operator (+) is also used for arithmetic addition • The function that the + operator performs depends on the type of the information on which it operates • If both operands are strings, or if one is a string and one is a number, it performs string concatenation • If both operands are numeric, it adds them • The + operator is evaluated left to right • Parentheses can be used to force the operation order • E.g.: Addition.java (p. 58)

11. Addition.java • //************************************************************* • // Addition.java Author: Lewis and Loftus • // Demonstrates the difference between the addition and string • // concatenation operators. • //************************************************************* • public class Addition • { • //----------------------------------------------------------------- • // Concatenates and adds two numbers and prints the results. • //----------------------------------------------------------------- • public static void main (String[] args) • { • System.out.println ("24 and 45 concatenated: " + 24 + 45); • System.out.println ("24 and 45 added: " + (24 + 45)); • } • }

12. Escape Sequences • What if we wanted to print a double quote character? • The following line would confuse the compiler because it would interpret the second quote as the end of the string System.out.println ("I said "Hello" to you."); • An escape sequence is a series of characters that represents a special character • An escape sequence begins with a backslash character (\), which indicates that the character(s) that follow should be treated in a special way System.out.println ("I said \"Hello\" to you.");

13. Escape Sequence \b \t \n \r \" \' \\ Meaning backspace tab newline carriage return double quote single quote backslash Escape Sequences • Some Java escape sequences: • E.g.: Roses.java (p. 59)

14. data type variable name Variables • A variable is a name for a location in memory • A variable must be declared, specifying the variable's name and the type of information that will be held in it int total; int count, temp, result; Multiple variables can be created in one declaration

15. Variables • A variable can be given an initial value in the declaration int sum = 0; int base = 32, max = 149; • When a variable is referenced in a program, its current value is used • E.g.: PianoKeys.java

16. PianoKeys.java PianoKeys.java • //************************************************************* • // PianoKeys.java Author: Lewis and Loftus • // Demonstrates the declaration, initialization, and use of an integer variable. • //************************************************************* • publicclass PianoKeys • { • //----------------------------------------------------------------- • // Prints the number of keys on a piano. • //----------------------------------------------------------------- • publicstatic void main (String[] args) • { • int keys = 88; • System.out.println ("A piano has " + keys + " keys."); • } • }

17. Assignment • An assignment statement changes the value of a variable • The assignment operator is the = sign total = 55; • The expression on the right is evaluated and the result is stored in the variable on the left • The value that was in total is overwritten • You can only assign a value to a variable that is consistent with the variable's declared type • E.g.: Geometry.java (p. 62)

18. Geometry.java Geometry.java • //*************************************************************** • // Geometry.java Author: Lewis and Loftus • // Demonstrates the use of an assignment statement to change the • // value stored in a variable. • //**************************************************************** • public classGeometry • { • // Prints the number of sides of several geometric shapes. • public static void main (String[] args) • { • int sides = 7; // declaration with initialization • System.out.println ("A heptagon has " + sides + " sides."); • sides = 10; // assignment statement • System.out.println ("A decagon has " + sides + " sides."); • sides = 12; • System.out.println ("A dodecagon has " + sides + " sides."); • } • }

19. Constants • A constant is an identifier that is similar to a variable except that it holds one value for its entire existence • The compiler will issue an error if you try to change a constant • In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 69; • Constants: • give names to otherwise unclear literal values • facilitate changes to the code • prevent inadvertent errors

20. Primitive Data • There are exactly eight primitive data types in Java • Four of them represent integers: • byte, short, int, long • Two of them represent floating point numbers: • float, double • One of them represents characters: • char • And one of them represents boolean values: • boolean

21. Type byte short int long float double Storage 8 bits 16 bits 32 bits 64 bits 32 bits 64 bits Min Value -128 -32,768 -2,147,483,648 < -9 x 1018 +/- 3.4 x 1038 with 7 significant digits +/- 1.7 x 10308 with 15 significant digits Max Value 127 32,767 2,147,483,647 > 9 x 1018 Numeric Primitive Data • The difference between the various numeric primitive types is their size, and therefore the values they can store:

22. Characters • A char variable stores a single character from the Unicode character set • A character set is an ordered list of characters, and each character corresponds to a unique number • The Unicode character set uses sixteen bits per character, allowing for 65,536 unique characters • It is an international character set, containing symbols and characters from many world languages • Character literals are delimited by single quotes: 'a' 'X' '7' '$' ',' '\n'

23. uppercase letters lowercase letters punctuation digits special symbols control characters A, B, C, … a, b, c, … period, semi-colon, … 0, 1, 2, … &, |, \, … carriage return, tab, ... Characters • The ASCII character set is older and smaller than Unicode, but is still quite popular • The ASCII characters are a subset of the Unicode character set, including:

24. Boolean • A boolean value represents a true or false condition • A boolean can also be used to represent any two states, such as a light bulb being on or off • The reserved words true and false are the only valid values for a boolean type boolean done = false;

25. Arithmetic Expressions • An expression is a combination of operators and operands • Arithmetic expressions compute numeric results and make use of the arithmetic operators: Addition + Subtraction - Multiplication * Division / Remainder % • If either or both operands to an arithmetic operator are floating point, the result is a floating point

26. Division and Remainder • If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded) 14 / 3 equals? 4 8 / 12 equals? 0 • The remainder operator (%) returns the remainder after dividing the second operand into the first 14 % 3 equals? 2 8 % 12 equals? 8

27. Operator Precedence • Operators can be combined into complex expressions result = total + count / max - offset; • Operators have a well-defined precedence which determines the order in which they are evaluated • Multiplication, division, and remainder are evaluated prior to addition, subtraction, and string concatenation • Arithmetic operators with the same precedence are evaluated from left to right • Parentheses can always be used to force the evaluation order

28. Operator Precedence • What is the order of evaluation in the following expressions? a + b + c + d + e a + b * c - d / e 1 2 3 4 3 1 4 2 a / (b + c) - d % e 2 1 4 3 a / (b * (c + (d - e))) 4 3 2 1

29. Assignment Revisited • The assignment operator has a lower precedence than the arithmetic operators First the expression on the right hand side of the = operator is evaluated answer = sum / 4 + MAX * lowest; 4 1 3 2 Then the result is stored in the variable on the left hand side

30. Assignment Revisited • The right and left hand sides of an assignment statement can contain the same variable First, one is added to the original value of count count = count + 1; Then the result is stored back into count (overwriting the original value)

31. Data Conversions • Sometimes it is convenient to convert data from one type to another • For example, we may want to treat an integer as a floating point value during a computation • Conversions must be handled carefully to avoid losing information • Widening conversions are safest because they tend to go from a small data type to a larger one (such as a short to an int) • Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short)

32. Data Conversions • In Java, data conversions can occur in three ways: • assignment conversion • arithmetic promotion • casting • Assignment conversion occurs when a value of one type is assigned to a variable of another • Only widening conversions can happen via assignment • Arithmetic promotion happens automatically when operators in expressions convert their operands

33. Data Conversions • Casting is the most powerful, and dangerous, technique for conversion • Both widening and narrowing conversions can be accomplished by explicitly casting a value • To cast, the type is put in parentheses in front of the value being converted • For example, if total and count are integers, but we want a floating point result when dividing them, we can cast total: result = (float) total / count;

34. Creating Objects • A variable either holds a primitive type, or it holds a reference to an object • A class name can be used as a type to declare an object reference variable String title; • No object has been created with this declaration • An object reference variable holds the address of an object • The object itself must be created separately

35. Creating Objects • We use the new operator to create an object title = new String ("Java Software Solutions"); This calls the Stringconstructor, which is a special method that sets up the object • Creating an object is called instantiation • An object is an instance of a particular class

36. Creating Objects • Because strings are so common, we don't have to use the new operator to create a String object title = "Java Software Solutions"; • This is special syntax that only works for strings • Once an object has been instantiated, we can use the dot operator to invoke its methods title.length()

37. String Methods • The String class has several methods that are useful for manipulating strings • Many of the methods return a value, such as an integer or a new String object • See the list of String methods on page 75 and in Appendix M • E.g.: StringMutation.java (p. 77)

38. StringMutation.java • //**************************************************************** • // StringMutation.java Author: Lewis and Loftus • // Demonstrates the use of the String class and its methods. • //**************************************************************** • public class StringMutation • { • //----------------------------------------------------------------- • // Prints a string and various mutations of it. • //----------------------------------------------------------------- • public static void main (String[] args) • { • String phrase = new String ("Change is inevitable"); • String mutation1, mutation2, mutation3, mutation4; • System.out.println ("Original string: \"" + phrase + "\""); • System.out.println ("Length of string: " + phrase.length());

39. StringMutation.java (cont.) • mutation1 =phrase.concat(", except from vending machines."); • mutation2 = mutation1.toUpperCase(); • mutation3 = mutation2.replace('E', 'X'); • mutation4 = mutation3.substring(3, 30); • // Print each mutated string • System.out.println ("Mutation #1: " + mutation1); • System.out.println ("Mutation #2: " + mutation2); • System.out.println ("Mutation #3: " + mutation3); • System.out.println ("Mutation #4: " + mutation4); • System.out.println ("Mutated length: " + mutation4.length()); • } • }

40. Class Libraries • A class library is a collection of classes that we can use when developing programs • There is a Java standard class library that is part of any Java development environment • These classes are not part of the Java language per se, but we rely on them heavily • The System class and the String class are part of the Java standard class library • Other class libraries can be obtained through third party vendors, or you can create them yourself

41. Purpose General support Creating applets for the web Graphics and graphical user interfaces Additional graphics capabilities and components Network communication Utilities Package java.lang java.applet java.awt javax.swing java.net java.util Packages • The classes of the Java standard class library are organized into packages • Some of the packages in the standard class library are:

42. The import Declaration • When you want to use a class from a package, you could use its fully qualified name java.util.Random • Or you can import the class, then just use the class name import java.util.Random; • To import all classes in a particular package, you can use the * wildcard character import java.util.*;

43. The import Declaration • All classes of the java.lang package are automatically imported into all programs • That's why we didn't have to explicitly import the System or String classes in earlier programs • The Random class is part of the java.util package • It provides methods that generate pseudo-random numbers • We often have to scale and shift a number into an appropriate range for a particular purpose • E.g. RandomNumbers.java (p. 82)

44. RandomNumbers.java • //**************************************************************** • // RandomNumbers.java Author: Lewis and Loftus • // Demonstrates the import statement, and the creation of pseudo- • // random numbers using the Random class. • //**************************************************************** • import java.util.Random; • public class RandomNumbers • { • // Generates random numbers in various ranges. • public static void main (String[] args) • { • Random generator = new Random(); • int num1; • float num2; • num1 = generator.nextInt(); • System.out.println ("A random integer: " + num1);

45. RandomNumbers.java (cont.) • num1 = Math.abs (generator.nextInt()) % 10; • System.out.println ("0 to 9: " + num1); • num1 = Math.abs (generator.nextInt()) % 10 + 1; • System.out.println ("1 to 10: " + num1); • num1 = Math.abs (generator.nextInt()) % 20 + 10; • System.out.println ("10 to 29: " + num1); • num2 = generator.nextFloat(); • System.out.println ("A random float [between 0-1]: " + num2); • num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 • num1 = (int) num2 + 1; • System.out.println ("1 to 6: " + num1); • } • }

46. Class Methods • Some methods can be invoked through the class name, instead of through an object of the class • These methods are called class methods or static methods • The Math class contains many static methods, providing various mathematical functions, such as absolute value, trigonometry functions, square root, etc. temp = Math.cos(90) + Math.sqrt(delta);

47. The Keyboard Class • The Keyboard class is NOT part of the Java standard class library • It is provided by the authors of the textbook to make reading input from the keyboard easy • Details of the Keyboard class are explored in Chapter 8 • For now we will simply make use of it • The Keyboard class is part of a package called cs1, and contains several static methods for reading particular types of data • E.g.: Echo.java (p. 86) • E.g.: Quadratic.java (p. 87)

48. Echo.java • //**************************************************************** • // Echo.java Author: Lewis and Loftus • // Demonstrates the use of the readString method of the Keyboard class. • //**************************************************************** • import cs1.Keyboard; • public class Echo • { • // Reads a character string from the user and prints it. • public static void main (String[] args) • { • String message; • System.out.println ("Enter a line of text:"); • message = Keyboard.readString(); • System.out.println ("You entered: \"" + message + "\""); • } • }

49. Quadratic.java • //**************************************************************** • // Quadratic.java Author: Lewis and Loftus • // Demonstrates a calculation based on user input. • //**************************************************************** • import cs1.Keyboard; • public class Quadratic • { • // Determines the roots of a quadratic equation. • public static void main (String[] args) • { • int a, b, c; // ax^2 + bx + c • System.out.print ("Enter the coefficient of x squared: "); • a = Keyboard.readInt(); • System.out.print ("Enter the coefficient of x: "); • b = Keyboard.readInt(); • System.out.print ("Enter the constant: "); • c = Keyboard.readInt();

50. Quadratic.java (cont.) • // Use the quadratic formula to compute the roots. • // Assumes a positive discriminant. • double discriminant = Math.pow(b, 2) - (4 * a * c); • double root1 = ((-1 * b) + Math.sqrt(discriminant)) / (2 * a); • double root2 = ((-1 * b) - Math.sqrt(discriminant)) / (2 * a); • System.out.println ("Root #1: " + root1); • System.out.println ("Root #2: " + root2); • } • }